1. Field of the Invention
The present invention relates to a lens barrel which is mountable on a camera, and is capable of transferring data stored in a ROM to the camera body through electrical contacts connected when the lens barrel is mounted on the camera.
2. Related Background Art
In photography, information on the phototaking distance, such as the distance to the main object or the phototaking magnification representing the size of the object on the image frame can be used for many purposes, such as exposure control in a flash phototaking operation and multi-pattern light metering for determining the exposure by individually measuring plural areas in the image frame. Such information is therefore useful for achieving multiple functions in the camera.
Japanese Patent Laid-Open Application No. 59-46631 has proposed a camera system in which data on the phototaking distance corresponding to the moving amount of a focusing member on the lens barrel are memorized in a ROH provided in the lens barrel, and, upon detection of the position of the focusing member, the corresponding data on the phototaking distance are transferred to the camera body.
In the above-mentioned prior art, the information has been described to represent the phototaking distance by data corresponding to the apex value of the set phototaking distance, but the specific data format or the method of exact transfer of said information on the phototaking distance to the camera body has not been clarified.
Valably, even if the absolute position of the focusing lens is detected and the data on the phototaking distance corresponding to said position are output, said data cannot be effectively utilized, because such data involve errors such as the error in the detection of an encoder, error in the manufacture of the optical system and fluctuation in the adjustment thereof.
In order to guarantee the accuracy of the data on phototaking distance in the lens barrel of the above-explained structure, the focal plane of the phototaking lens system has always to coincide with a predetermined focal plane (film plane).
However, in various cases, the focal plane of the phototaking lens system becomes aberrated (deviated) from the film plane and the accuracy of the data on the phototaking distance is deteriorated.
The following cases are illustrative:
(1) Positional fluctuation of the phototaking lens system by the FFD (flange-focus distance) adjustment; PA1 (2) Focus displacement by zooming; PA1 (3) Focus displacement caused by lens expansion or contraction resulting from temperature change; and PA1 (4) Variation in aberrations depending on phototaking distance.
In each of the above cases, the actual focal plane of the phototaking lens becomes aberrated from the designed focal plane (film plane), so that an error is generated between the phototaking distance data memorized in the ROM and the actual phototaking distance.
Regarding case (1), FFD refers to the distance between a predetermined image plane of the optical system (film plane in case of camera) and a reference position of the lens (reference face of the bayonet mount in case of camera), and the FFD adjustment means the adjustment of the lens position in such a manner that the light beam from the infinite distance is focused on the predetermined image plane.
For example, for a target value of FFD=A, the lens before adjustment is generally in a state FFD=A+.alpha. and an adjustment .alpha..fwdarw.0 is conducted. However, since it is practically difficult to bring .alpha. to zero completely, the adjustment is executed into a range FFD=A.+-.dA.
In the above-mentioned four cases, the variation of phototaking distance per unit amount of movement of the phototaking optical system is particularly large in the vicinity of infinite distance, so that the error in the phototaking distance data becomes very large.
Also the error mentioned in case (1) cannot be corrected in advance in uniform manner, because the lens barrels have individual fluctuations within the tolerance of FFD adjustment.